1. Field of the Invention
The present invention is generally related to broadband communications systems. More particularly, the present invention is related to broadband communications systems that use Data Over Cable Service Interface Specification (DOCSIS) or any of its derivatives.
2. Background Art
In DOCSIS related broadband communications architectures, data is transferred between a central location and many remote subscribers. The central location may be referred to as a headend for cable systems, a wireless access termination system (WATS) for broadband terrestrial fixed wireless systems, or a satellite gateway for two-way satellite systems. Subscriber equipment may be referred to as a cable modem (CM) for cable systems, a wireless modem (WM) for broadband terrestrial fixed wireless systems, or a satellite modem (SM) for two-way satellite systems.
In a two-way satellite system, the communication path from the satellite gateway to the SM is called the downstream and the communication path from the SM to the satellite gateway is called the upstream. In standard DOCSIS based systems, the upstream is implemented in a time division multiple access (TDMA) protocol with a fixed modulation type and forward error correction (FEC) coding rate. Such signals have a fixed spectral efficiency in bits per second/Hertz (bps/Hz). Signal parameters such as modulation type, FEC coding type, and FEC coding rate determine the signal-to-noise ratio (SNR) required for the SM to have error-free or quasi error-free operation in a given channel. There is an inherent trade-off between receiver parameters that allow for high throughput (high order modulation, high FEC code rates) and those that allow the signal to be reliably received at low SNRs, but with a lower throughput (low order modulations, robust low code rate FECs).
In many real world environments, subscribers experience a wide range of path losses and channel degradations. For example, in the case of a satellite based system where a downstream spot beam is broadcasting to SMs that are located over a wide geographic area, various degradations, such as localized rainfall, partial obstructions, antenna misalignments, etc., cause the upstream signal power levels and SNRs received at the satellite gateway from individual subscribers to vary significantly. Terrestrial wireless and cable systems may experience the same phenomenon for different reasons.
Current DOCSIS based systems operate with parameters that allow the worst case subscriber to obtain service with a given probability of success. Subscribers that could otherwise transmit upstream data at a higher rate are penalized by the presence of disadvantaged subscribers.
Thus, what is needed is a system and method of dynamically assigning data traffic with different modulation orders and FEC parameters to different SMs within the same upstream channel, referred to as “upstream adaptive modulation (US-AM).” What is also needed is a system and method that implements US-AM in a manner that enables non US-AM enabled SMs to efficiently continue operation.